This invention relates to a circuit for controlling a signal level and more particularly to a volume control circuit suitable for a low distortion type audio equipment.
Generally, volume control and balance control functions are necessary for an audio equipment, e.g. a radio receiver for stereo broadcasting or a stereophonic playback apparatus. Many control circuits having these functions have been proposed. For such control circuit, it is desirable that the circuit, especially a volume control circuit, has a low distortion characteristic and is suitable for an integrated semiconductor circuit.
FIG. 1 shows a preceding invention of a volume control circuit which is disclosed in a U.S. patent application (U.S. Ser. No. 275,155, filed on June 19, 1981, inventors are the same as that of this application). The present invention has been made for improving this preceding invention, but the preceding one is not a prior art harming the novelty and inventive step of the present invention.
Referring now to FIG. 1, an audio signal Vs at an input terminal 20 is applied to a differential amplifier 21 via a coupling capacitor C21. The differential amplifier 21 comprises first and second transistors Q21 and Q22. The base of first transistor Q21 is connected to the capacitor C21 and to a positive terminal of a fixed DC voltage source V21 through a resistor R21. The negative terminal of voltage source V21 is grounded. The emitter of first transistor Q21 is grounded through a constant current source 22. The base of second transistor Q22 is also connected to the positive terminal of voltage source V21 through a resistor R22. The emitter of second transistor Q22 is grounded through a constant current source 23 and to the emitter of first transistor Q21 through a resistor R23.
The audio signal, after being amplified by the differential amplifier 21, is supplied from the collector of each of first and second transistors Q21 and Q22 to a substantially doubly balanced amplifier 24 which controls the gain of the audio signal. The doubly balanced differential amplifier 24 comprises third to sixth transistors Q23 to Q26. The emitter of third and fourth transistors Q23 and Q24 are connected in common to the collector of first transistor Q21. The emitter of fifth and sixth transistors Q25 and Q26 are connected in common to the collector of second transistor Q22. The base of third and sixth transistors Q23 and Q26 are connected in common to a positive terminal of a variable DC voltage source V22, the negative terminal of which is grounded. The base of fourth and fifth transistors Q24 and Q25 are connected in common to a positive terminal of a fixed DC voltage source V23, the negative terminal of which is grounded.
A controlled audio signal is supplied from the collector of each of third and sixth transistors Q23 and Q26 to a current mirror circuit 25. The collector of fourth and fifth transistors Q24 and Q25 are connected in common to a line L21 of a power supply source Vcc. Accordingly, another controlled audio signal of transistors Q24 and Q25 is bypassed to line L21. The current mirror circuit 25 comprises seventh and eighth transistors Q27 and Q28. The emitter of seventh transistor Q27 is connected to the collector of third transistor Q23 and to line L21 through a resistor R24. The emitter of eighth transistor Q28 is connected to the collector of sixth transistor Q26 and to line L21 through a resistor R25. The base of eighth transistor Q28 is connected to the base of seventh transistor Q27. The collector of eighth transistor Q28 is connected to its base and to the circuit ground through a constant current source 26.
The controlled audio signal is supplied from the collector of seventh transistor Q27 to an output terminal 29. The collector of seventh transistor Q27 is grounded via a resistor R26 which forms a load circuit 27. The controlled audio signal Vo is provided from the output terminal 29 to a loudspeaker (not shown).
The operation of the above described volume control circuit is as follows. When an audio signal having a value Vs is applied to the differential amplifier 21 via input terminal 20 and capacitor C21, the value of the audio signal current flowing through resistor R23 is expressed as follows: ##EQU1## where Ra is the resistance of resistor R23.
Accordingly, if the current of constant current source 22 is equal to that of constant current source 23 and is expressed as I01, a value of the audio signal current at the collector of first transistor Q21 is (I01+i.sub.s) and the value of the audio signal current at the collector of second transistor Q22 is (I01-i.sub.s)
The audio signal current from each of first and second transistors Q21 and Q22 is supplied to doubly balanced amplifier 24 to control the gain of the audio signal. The control function of doubly balanced differential amplifier 24 is achieved by adjusting the potential value E22 of voltage source V22 in relation to the potential value E23 of voltage source V23. When value E22 is larger than value E23, and value E22 is increased until third and sixth transistors Q23 and Q26 conduct and fourth and fifth transistors Q24 and Q25 are cut off, the audio signal current from the collector of each of first and second transistors Q21 and Q22 flows only through the collector and emitter path of each of third and sixth transistors Q23 and Q26, respectively. Accordingly, the gain of the audio signal, that is, the volume, is maximized. On the other hand, when value E22 is smaller than value E23, and value E22 is decreased until third and sixth transistors Q23 and Q26 are cut off and fourth and fifth transistors Q24 and Q25 conduct, the audio signal current from the collector of each of first and second transistors Q21 and Q22 flows only through the collector and emitter path of each of fourth and fifth transistors Q24 and Q25, respectively. Accordingly, the gain of the audio signal is minimized.
The maximum gain of the audio signal in the volume control circuit is calculated below. When seventh and eighth transistors Q27 and Q28 constitute current mirror circuit 25 having a current ratio 1, the value of the controlled audio signal current flowing through resistor R24 is equal to that of the controlled audio signal current flowing through resistor R25, where the resistance of resistor R24 is equal to that of resistor R25. That is, EQU I02+I01-i.sub.s =Ic+I01+i.sub.s (2)
where I02 is the current of constant current source 26 and Ic is the controlled audio signal current supplied from the collector of seventh transistor Q27.
From the equation (2), Ic can be calculated as follows: EQU Ic=I02-2i.sub.s (3)
Accordingly, the output voltage Vo of the controlled audio signal at output terminal 29 is expressed as follows: EQU Vo=(I02-2i.sub.s)(Rb), (4) where Rb is the resistance of resistor R26.
The AC component of the output voltage Vo is as follows: EQU Vo=(2i.sub.s)(Rb) (5)
As a result, the maximum gain G.sub.max of the audio signal is expressed as follows: ##EQU2##
In the above circuit, the volume control is not influenced by fluctuations in the power source Vcc. Even if the power source in line L21 includes a fluctuation component such as a ripple, the bias voltage across the base and emitter of each of seventh and eighth transistors Q27 and Q28 is unvaried because seventh and eighth transistors Q27 and Q28 constitute the current mirror circuit 25 with constant current source 26. Accordingly, the controlled audio signal from the collector of seventh transistor Q27 does not include the fluctuation element or ripple of power source Vcc. The volume control circuit also does not generate an audio component when the volume control is adjusted to the minimum. Since both the collectors of fourth and fifth transistors Q24 and Q25 are connected to line L21, the controlled audio signal current flowing through the collector and emitter path of each of fourth and fifth transistors Q24 and Q25 is completely bypassed to line L21. Further, when the volume control circuit is integrated into a semiconductor, and the circuit ground is connected through a common ground line to the outside of the semiconductor, the current flowing from the constant current sources 22 and 23 into the ground line is unvaried and is independent of the audio input signal. Accordingly, the potential in the circuit ground line does not fluctuate. As a result, the volume control circuit does not generate an audio component when the volume is minimum.
The circuit of FIG. 1 has advantagious features as mentioned. However, as to the distortion characteristic, there is a possibility of improvement. That is, since the both emitter currents of transistors Q27 and Q28 forming current mirror circuit 25 are varied with the change of signal current i.sub.s, the base-emitter voltage VF of each of transistors Q27 and Q28 is subjected to vary with the change of i.sub.s. Accordingly, the current flowing through load resistor R26 contains not only the current i.sub.s but a noise current caused by the i.sub.s change. Thus, the distortion factor of output signal obtained from terminal 29 is liable to somewhat worsen due to the noise current.